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Financial Instruments - Sensitivity ranges for the base fair values (Details) | 12 Months Ended |  |
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Dec. 31, 2020 CAD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 CAD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Long-term power sale – US | Long-term contracts |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Base fair value | $ 598,000,000 |  | $ 737,000,000 |  |  |
Coal transportation – US |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Base fair value | (16,000,000) |  |  |  |  |
Structured products – Eastern US |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Base fair value |  |  | 7,000,000 |  |  |
Full requirements – Eastern US |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Base fair value | 11,000,000 |  | 10,000,000 |  |  |
Full requirements – Eastern US | Cost Of Supply |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  | $ 1 |  | $ 1 |  |
Long-term wind energy sale –Eastern US |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Base fair value | (29,000,000) |  |  |  |  |
Long-term wind energy sale –Eastern US | Long-term contracts |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Base fair value |  |  | (28,000,000) |  |  |
Other |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Base fair value | (4,000,000) |  | (6,000,000) |  |  |
Fixed-price contracts | Long-term contracts | Level III |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Future power price ranges per MWh |  |  |  |  |  | $ 1
Increase (decrease) in derivative future power prices, reasonable possible |  | (1) |  |  |  |
Forward contract | Long-term contracts | Level III |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Future power price ranges per MWh |  |  |  |  |  | $ 6
Increase (decrease) in derivative future power prices, reasonable possible |  | (6) |  |  |  |
Minimum | Long-term power sale – US | Long-term contracts |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | 59,000,000 |  | 139,000,000 |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  | (3) |  | $ 3 |  |
Minimum | Coal transportation – US |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | 5,000,000 |  |  |  |  |
Minimum | Coal transportation – US | Long-term contracts |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  | (3) |  |  |  |
Minimum | Coal transportation – US | Long-term contracts | US |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Future power price ranges per MWh |  |  |  |  | $ 24 |
Rail rate escalation | $ 21 |  |  |  |  |
Increase (decrease) in rail rate escalation expressed as a percentage | 0 |  |  |  | 0 |
Minimum | Coal transportation – US | Long-term contracts | US | Historical volatility for shares, measurement input [member] |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Significant unobservable inputs, derivative asset | 0.15 |  |  |  | 0.15 |
Increase (decrease) in significant unobservable inputs, derivative asset | 0.80 |  |  |  | 0.80 |
Minimum | Structured products – Eastern US |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments |  |  | $ 2,000,000 |  |  |
Basis relationship, percent |  |  | 91.00% | 91.00% |  |
Non-standard shape factors, percent |  |  | 63.00% | 63.00% |  |
Sensitivity analysis in derivative, basis relationship, percent |  |  | 4.00% | 4.00% |  |
Sensitivity analysis in derivative, non-standard shape factors, percent |  |  | 4.00% | 4.00% |  |
Minimum | Full requirements – Eastern US |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 3,000,000 |  | $ 3,000,000 |  |  |
Minimum | Full requirements – Eastern US | US | Volume Rate |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Increase (decrease) in significant unobservable inputs, derivative asset | 0.95 |  | 0.95 |  | 0.95 | 0.95
Minimum | Long-term wind energy sale –Eastern US |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 22,000,000 |  |  |  |  |
Minimum | Long-term wind energy sale –Eastern US | Long-term contracts |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments |  |  | $ 20,000,000 |  |  |
Minimum | Other |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | 5,000,000 |  | 8,000,000 |  |  |
Minimum | Fixed-price contracts | Long-term contracts | US |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Future power price ranges per MWh |  |  |  |  | $ 24 | $ 20
Minimum | Forward contract | Long-term contracts | Level III |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Future power price ranges per MWh |  |  |  |  | 35 | $ 38
Maximum | Long-term power sale – US | Long-term contracts |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | 35,000,000 |  | 46,000,000 |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  | 5 |  | $ 9 |  |
Maximum | Coal transportation – US |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | 3,000,000 |  |  |  |  |
Maximum | Coal transportation – US | Long-term contracts |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  | $ 5 |  |  |  |
Maximum | Coal transportation – US | Long-term contracts | US |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Future power price ranges per MWh |  |  |  |  | $ 32 |
Rail rate escalation | $ 24 |  |  |  |  |
Increase (decrease) in rail rate escalation expressed as a percentage | 0.04 |  |  |  | 0.04 |
Maximum | Coal transportation – US | Long-term contracts | US | Historical volatility for shares, measurement input [member] |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Significant unobservable inputs, derivative asset | 0.40 |  |  |  | 0.40 |
Increase (decrease) in significant unobservable inputs, derivative asset | 1.20 |  |  |  | 1.20 |
Maximum | Structured products – Eastern US |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments |  |  | $ 2,000,000 |  |  |
Basis relationship, percent |  |  | 112.00% | 112.00% |  |
Non-standard shape factors, percent |  |  | 116.00% | 116.00% |  |
Sensitivity analysis in derivative, basis relationship, percent |  |  | 6.00% | 6.00% |  |
Sensitivity analysis in derivative, non-standard shape factors, percent |  |  | 10.00% | 10.00% |  |
Maximum | Full requirements – Eastern US |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 3,000,000 |  | $ 3,000,000 |  |  |
Maximum | Full requirements – Eastern US | US | Volume Rate |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Increase (decrease) in significant unobservable inputs, derivative asset | 1.05 |  | 1.05 |  | 1.05 | 1.05
Maximum | Long-term wind energy sale –Eastern US |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 22,000,000 |  |  |  |  |
Maximum | Long-term wind energy sale –Eastern US | Long-term contracts |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments |  |  | $ 20,000,000 |  |  |
Maximum | Other |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 5,000,000 |  | $ 8,000,000 |  |  |
Maximum | Fixed-price contracts | Long-term contracts | Level III |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Future power price ranges per MWh |  |  |  |  | $ 11 | $ 9
Maximum | Fixed-price contracts | Long-term contracts | US |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Future power price ranges per MWh |  |  |  |  | 32 | 28
Maximum | Forward contract | Long-term contracts | Level III |  |  |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |  |  |
Future power price ranges per MWh |  |  |  |  | $ 52 | $ 60

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